High temperature sealants and adhesives are valuable industrial materials. There is a continuing search for materials which are useful at ever increasing temperatures and which maintain useful properties. For example, in autoclave curing and processing of thermosetting and thermoplastic composites, sheer strength, flexibility and thermal stability of the applied vacuum bag sealant are important characteristics. Generally, vacuum bag sealants have been based on butyl rubber and polydimethylsiloxanes. The use of these materials establishes a maximum processing temperature of about 650.degree. to 700.degree. F. Above these temperatures, these sealants degrade, char and lose their ability to maintain an airtight seal.
During this curing process, vacuum is periodically applied through a port in the vacuum bag. While vacuum is being applied, the vacuum bag sealant must maintain an airtight seal. In prior art processes, the sealant frequently fails before the end of the processing cycle, particularly when temperatures reach 700.degree. F. and above. This results in a loss of vacuum in the vacuum bag and the composite part being made is then a total loss.
Phenylsiloxanes have been incorporated in pressure sensitive adhesives to attain useful adhesives to temperatures of less than about 550.degree. F. For example, U.S. Pat. No. 3,032,438 to Gaynes, et al. discloses a pressure sensitive adhesive tape made from a backing material and two layers of a cured silicone. This is accomplished by applying a base layer of an adhesive consisting essentially of a silicone resin (an organo-silicone polymer identified as a mixture comprising principally methyl, phenyl, and methyl-phenyl polysiloxanes), about 0.5 to 5 parts by weight per 100 parts resin of a benzoyl peroxide curing agent and up to about 30 parts by weight per 100 parts resin of a filler such as hydrated alumina, titanium dioxide, zinc oxide, calcium carbonate, asbestos and similar materials, in a solvent such as toluol, heptane, xylene, or benzol to a fiberglass cloth backing. This layer is dried to drive off the solvent and subsequently coated with a second layer. The second layer is a solution of a silicone resin and from about 0.5 to about 5 parts of a curing agent consisting of tri-n-butylamine. The solvent is again driven off in an oven. The adhesive tape is then cured to provide a pressure sensitive adhesive tape effective for use within a temperature range of about -110.degree. to 550.degree. F. Thus, Gaynes discloses a cured, pressure sensitive adhesive tape which is formed in a complicated, two-step process using solvent-based application of the adhesive. The resulting adhesive apparently would not be useful in autoclave operations at temperatures greater than 700.degree. F.
U.S. Pat. No. 3,527,842 to Clark discloses a pressure sensitive adhesive useful for bonding films, such as polyamide films, to metal substrates, such as electrical conductors. The adhesive is a composition which consists essentially of (1) a methylphenylpolysiloxane resin, (2) a polysiloxane containing from 0.15 to 0.60 vinyl radicals per silicon atom, (3) a siloxane compatible with (1) and (2) having at least two SiH groups per molecule on the average, the remaining valences of the silicon atoms in the siloxane (3) being essentially all satisfied with aryl hydrocarbon and methyl radicals and SiOSi linkages, and (4) a catalyst for promoting the reaction between (2) and (3). This adhesive is apparently stable to a temperature of about 500.degree. F. In sum, Clark discloses a pressure sensitive adhesive composition comprising a mixture of polysiloxane resins and a catalyst which promotes a cross linking reaction between two of the resins. The resulting adhesive apparently would not be useful in autoclave operations at temperatures greater than 700.degree. F.
Accordingly, a substantial need exists for a polyorganosiloxane sealant or adhesive which is stable and which does not degrade or lose tack and adhesion at temperatures greater than about 700.degree. F.